An oscilloscope is a wonderful tool for discovering how analog electrical signals behave with the passage of time. Particularly so for signals that we classify as ‘rapid,’ by which we mean that the variation in the signal is too fast for strip chart recorders and data loggers. Thus, we tend to think of recorders and loggers as tools for ‘steady state’ signals whose values may be expected to change as required by external conditions, but not for most signals that have ‘waveforms.’ Indeed, the principle excellence of oscillographic techniques is that they allow us to ‘see’ events whose time scales are enumerated in milliseconds, microseconds, or, nanoseconds. Thus it is that analog oscilloscopes have selectable horizontal ‘sweep speeds’ calibrated in terms of time/division for a graticule on the CRT's faceplate.
Often, what was required from early oscilloscopes was information about the amplitude or shape of a truly cyclic signal, such as produced by an oscillator, or received from an antenna or microphone, and then amplified. The two and three inch round CRTs (Cathode Ray Tubes) of early ‘scopes were thought large enough to adequately reveal such information for one to, say, five, cycles of such signal behavior. By and large, it was thought sufficient for one instance of a signal's cycle to represent the others, and unique or unusual sequences of disparate events that were too long to fit onto the viewing area of the CRT as individually recognizable events were problematic: the sweep speed needs to be at least as fast as required to resolve the shortest event into a recognizable portion of the trace, while the length needed for a trace is determined by the separation between events in the sequence, but the CRT's size limits the length of that trace. It sometimes happened that the sweep speed that produced a trace that encompassed all the events of interest was slow enough that events in the trace were crowded together to the extent that the events could not be distinguished from one another. CRTs of five inch diameter (and various rectangular sizes) later came into widespread use, and while that was a welcome development (the traces weren't so tiny, anymore), it scarcely solved the crowding problem described above.
The community of ‘scope manufacturers was first obliged to provide an X5 and an X10 control that amplified the sweep voltage applied to the horizontal deflection plates, resulting in most of the trace being off screen. The horizontal position control determined what portion was on screen, and allowed the operator to ‘pan’ along the whole trace, provided it was recurrent, and stable, etc. This allowed an expanded view of events that were too crowded to be distinguished at the normal ‘X1’ setting. But if a ‘scope had to emulate a strip chart recorder for an extended high speed event that occurred just once or infrequently (studies of nuclear explosions come to mind), a moving film camera was fitted to the CRT and the ‘scope’s internal sweep was disabled.
Later, when triggered sweeps became common, the notion of delayed sweep allowed a more elegant solution than simple horizontal magnification (which nevertheless remained on the front panels of most ‘scopes). With delayed sweep a trigger initiates a variably selectable delay afterwhich the sweep is performed to create a trace. Panning is now accomplished by varying the calibrated delay.
As digital computation and digital control mechanisms became more pervasive the notion that one cycle of a signal was as good as any other became less applicable, and ‘scope users in these new digital applications became creative in how to make the best use of the techniques just described to create the needed traces for sequences of disparate events. It was thus a welcome development when the DSO (Digital Sampling Oscilloscope) arrived with its memory.
The fact that a DSO has memory gives it at least two distinct advantages over its analog predecessor. The first of these is related to bandwidth. It turns out that to get all the components in the vertical path of an analog ‘scope to perform at high bandwidths is a significant engineering challenge. DC coupled amplifiers that will produce a hundred or more volts peak to peak at several gigahertz at a CRT's vertical deflection plates are not practical, not to mention that the writing rate for a normal CRT does not go that high. Even before the DSO, the highest frequency analog ‘scopes were sampling analog oscilloscopes (as opposed to ‘real time’ analog oscilloscopes) that relied upon regularly spaced (analog!) samples taken upon a repetitive waveform to recreate on the CRT an analog image of the input waveform. These analog samples (say, the charge on a tiny capacitance acquired during a very brief duration), when considered in sequence, formed a ‘slow moving’ analog voltage replica of the (‘fast moving’) applied input voltage. What the DSO does is take the sample and digitize it, and then store it in an Acquisition Memory operated as if it were a circular buffer. (The DSO might take the samples consecutively within a segment of an applied signal and at a very high rate for ‘real time’ operation or for ‘single shot’ operation, or it might let locations sampled at a slower rate drift across repeated cycles of the input signal for ‘repetitive sampling,’ also called ‘equivalent time’ operation.) It is then evident that once the digitized values are stored, they can be ‘played back’ at a convenient rate from a Frame Buffer using low cost raster scan techniques that are not affected by the possible high frequency (say, 20 GHz or more) nature of the applied input signal. The underlying technical issue here is that it is far easier to design and build high speed samplers and ADCs (Analog to Digital Converters) and fashion a high speed path into memory (say, by interleaving many banks of memory) than it is to design and build the equivalent actual analog signal path (amplifiers and CRT).
The second distinct advantage of the DSO over its analog parent arises because of the persistence of memory. Whereas the analog ‘scope was forced to “view the signal end-on, process it in real time and get rid the fleeting result” right away, the DSO “views the signal end-on but creates a ‘side view’ of its activity over a segment of time that is ‘permanent’” and that can be leisurely, as it were, processed, viewed and otherwise given a suitable disposition.
The notion that the signal's waveform is represented by a collection of digitized values in a memory allows a powerful extension of the notion of triggering. Whereas the analog ‘scope could only unblank the beam and start the sweep subsequent to the occurrence of a trigger, the DSO can allow the operator to decide where the trigger event is to be relative to the start and end of the Acquisition Record. So, for example, if the creation of the Acquisition Record is continued until it is about to overwrite in the (circular) Acquisition Memory the location corresponding to (or most nearly corresponding to) the time when the trigger event occurred, then the Acquisition Record will produce a trace of activity occurring subsequent to the trigger, just as for analog ‘scopes. But if the creation of the Acquisition Record is stopped immediately upon the occurrence of the trigger event, then what the Acquisition Record contains is the activity that lead up to the trigger (so called ‘negative time’). This can be an invaluable feature that simply isn't possible with the old analog architecture, and we may speculate that this, in conjunction with the bandwidth issue, is what accounts for the decline in popularity of the ‘laboratory grade’ analog oscilloscope in favor of the modem DSO. If the creation of the Acquisition Record is continued for, say, half its length, then we have captured activity both before and after the triggering event.
Now let the Acquisition Record be substantial in size, perhaps large enough that it is apparently very many times wider than the Frame Buffer. A Frame Buffer might have, say, just one or two thousand addressable locations, because the physical display device has just that many horizontal pixel locations. But if the Acquisition Record has several million (or several tens of millions) of addressable locations, then there arises the issue of how to decide what image is to be stored in the Frame Buffer.
The operator may decide to ‘zoom out’ and let the end points of the Frame Buffer correspond to the start and end of the Acquisition Record. (Recall that the Acquisition Memory is managed as though it were a circular buffer, so those starting and ending locations in the resulting Acquisition Record are generally nearly adjacent, and located ‘anywhere along the circle,’ as it were.) The resulting displayed trace is, of necessity, severely compressed along the horizontal (time) dimension, and some clever rendering techniques are often required to create a useful image that is not downright deceptive and that correctly conveys some general sense of what signal activity is actually going on.
On the other hand, the operator may decide to view just a segment of the total Acquisition Record, and at a time scale selected from among predetermined choices. That is, within certain limits, the operator can both zoom and pan along the horizontal axis. This kind of operation has become (after the eventual emergence of user friendly controls to support it) the distinguishing hallmark of the DSO: those stuck with older analog equipment could only view with envy the measurements that their more newly equipped brethren could perform.
With this kind of flexibility comes some inevitable complexity. In this case, we can have what amounts to ‘an entire strip chart's worth of data’ but we still have just a tiny screen to view it on, and various techniques have emerged to help locate, and navigate back and forth between, separate events of interest in the ‘whole trace.’ The aggravation and chances for error associated with present navigation techniques are bound to become exacerbated with time, as DSOs with hundreds of megabytes of memory, and even memories in the gigabyte range, are poised to enter the marketplace. This situation will become one where a very long and detailed trace of a signal's waveform can be represented by the Acquisition Record (say, ten or one hundred times what is presently on the market). Besides simply panning along the trace with a manual control, there are configurable automatic tools to discover the existence of potential events of interest. However discovered, the locations in the Acquisition Record of such events of interest can be somehow marked (as with Bookmarks described in U.S. Pat. No. 6,958,754 B2, by Alexander & Oldfield) or their locations otherwise remembered with indexes that point to their locations. Alexander and Oldfield even provide a mechanism to go from one bookmark to any other.
The bookmark technique of Alexander and Oldfield requires the operator to manually establish a bookmark to represent an event of interest, and while it allows the association of a name and comment with a bookmark, the underlying indexing scheme is one based on the order in which the bookmarks are defined (which might be arbitrary), rather than the natural order of succession of the bookmarked events in the trace. So, when visiting bookmarks, and going from one to another, it is entirely up to the operator to ensure that his visits match the order of the succession of events (or of any other ordering), if that is his intent, as there is nothing inherent in the bookmark concept to support an ordering other than that with which the bookmarks were defined (within which ordering the notions of ‘NEXT’ and ‘PREVIOUS’ are indirectly available).
In some circumstances there are likely to be too many instances (say, thirty, fifty, a hundred?) of such events of interest to easily keep track of. What is needed is a way to easily and quickly navigate between (visit, and revisit) such a large number of events. This is particularly so in the case where the events to be visited are found by an automatic discovery mechanism (e.g., an automated measurement subsystem is asked to find everywhere that the rise time is greater than some amount, or wherever a transition in a selected direction does not achieve a minimum threshold). In such cases, the operator is asking the ‘scope to find such locations, say, using automated measurement techniques set out and described (among other places) in U.S. patent application Ser. No. <unknown> filed 31 May 2006 and entitled COMPOSITE TRIGGER FOR A DIGITAL SAMPLING OSCILLOSCOPE. So, let's say that the ‘scope found fifty-seven events in a really long trace that met some criteria that piques our interest. The automated measurement subsystem can scan the Acquisition Record to detect satisfactions of selected criteria, and even though it might tell us the number of such events, their minimums, maximum and averages, etc., it does not ‘create a trail of bookmarks,’ as it were. It is still up to the operator to manually direct the ‘scope to display the trace segment for each of the events that met the criteria. We should like to do the next step: Quickly and easily view the fifty-seven events in the order they occurred, or, perhaps instead in the order of their severity; all the better to determine if any of them are worthy of continued interest and increased scrutiny. How to do it?